Sodium pump stoicheiometry determined by simultaneous measurements of sodium efflux and membrane current in barnacle

Ouabain-sensitive Na efflux, membrane potential and membrane current were measured in single, perfused muscle cells taken from the giant barnacle, Balanus nubilus. This preparation permits control of the intracellular and extracellular solution composition as well as control of the membrane potential while measuring ion fluxes across the plasma membrane. The addition of ouabain (10(-4) M) to the extracellular solution produces a rapid depolarization of membrane potential (1-4 mV) and a simultaneous and proportional reduction of Na efflux (10-40 pmol/s). Ouabain-induced changes in membrane potential or Na efflux do not depend on the presence of extracellular Na. Under voltage control, the application of ouabain (10(-4) M) produces a rapid monotonic fall in outward current (1-3 microA) and a simultaneous fall in Na efflux (10-40 pmol/s). The fraction of ouabain-dependent Na efflux that appears as outward current is constant in any given preparation as the Na-pump turnover rate varies. Over a limited range, changes in membrane potential do not affect ouabain-sensitive Na efflux. The ouabain-sensitive Na efflux and membrane current are not altered by the presence of 50 mM-internal tetraethylammonium (TEA) ions. We conclude that the Na pump is electrogenic in barnacle muscle and that 49 +/- 10% of the extruded Na+ leaves the intracellular compartment as uncompensated charge. We find that the transport stoicheiometry of Na:K, calculated from the ouabain-dependent changes in membrane current and Na efflux, is between 3:2 and slightly more than 2:1.